The invention is in the general field of electric motor control. Specifically, the invention uses a Triac and a phase locked loop to control the amplitude of reciprocation of a linear motion AC motor, in such a way that pulses of power line noise have little effect on amplitude or phase of reciprocation.
Triac controllers (Ref. 2) offer an inexpensive means of controlling reciprocation amplitude of linear motion AC motors (Ref. 1), but in prior art have the disadvantage of responding to pulses of electrical noise on the AC power line if peak pulse voltage is high enough to cause the power line voltage to cross zero volts. A spurious zero crossing is treated by the prior art control as a normal zero crossing, and, since prior art Triac control generates Triac firing pulses that are timed relative to zero crossings of power line voltage, the result of a pulse of line noise can be generation of a spurious Triac firing pulse. If the linear motor drives a free piston machine such as a Stirling refrigerator, incorrectly timed Triac firing can cause damage to the machine and/or loss of amplitude control. Electrical filtering of line voltage can attenuate line noise, but suppression by filtering of pulses of line noise occurring very near normal zero crossings, to the point where a spurious zero crossing cannot occur, is impractical.
The invention uses a Triac and a phase locked loop to control the amplitude of reciprocation of a linear motor with substantial isolation from disturbance by line noise pulses. Amplitude control in the invention is achieved, as in prior art, by connecting the series combination of the motor and a Triac across an AC power source, and varying the phase of Triac firing pulses relative to the power source voltage. The invention departs from prior art in that firing pulses are generated at transitions of the square wave output of a Voltage Controlled Oscillator (VCO) that is part of a phase locked loop (PLL). The VCO is synchronized in frequency with the AC power source by the PLL, but is practically isolated from power line noise pulses. In the invention, the phase of the VCO output and therefore of the Triac firing pulses, relative to line voltage, is varied with a control voltage that is proportional to the difference between a first voltage proportional to an xe2x80x9camplitude set-pointxe2x80x9d voltage and a second voltage that is proportional to a measured value of reciprocation amplitude. A decrease in measured amplitude causes a leading phase shift of firing pulses, which constitutes negative feedback that acts to hold the amplitude of reciprocation of the motor at a preset value determined by the amplitude set point voltage.
In one form of the invention, a voltage proportional to the amplitude of reciprocation of the linear motor is derived by analog or digital computation based on the equivalent circuit of the linear motor, thus obviating the cost and complexity of a position sensor.
The basic components of the phase locked loop used in the invention can be elements of a single integrated circuit (e.g., type 4046, see Ref 4).